Functional residual capacity (FRC) is best described as which of the following?

• A. Total volume of gas in the lungs and airways
• B. Total volume of gas in the thorax at the end of expiration
• C. Volume of gas in the lungs that is in direct communication with the airways at the end of expiration
• D. Volume of gas in the lungs that remains in the alveoli after expiration

Answer: (C)

Functional residual capacity is the amount of air remaining in the lungs after a normal expiration. It sits at the balance point between the elastic recoil of the lungs and the chest wall, so it represents the end-expiratory lung volume. The key idea is that this gas stays in the lungs and remains in direct communication with the airways, meaning it’s the air that can participate in the next breath. In healthy lungs this volume equals the expiratory reserve volume plus the residual volume, rather than representing all gas in the thorax or gas isolated from the airways.